In Parshas Re'eh we read (Deuteronomy 14:1-2):
בנים אתם לה' אלקיכם לא תתגדדו ולא תשימו קרחה בין עיניכם למת: כי עם קדוש אתה לה' אלקיך ובך בחר ה' להיות לו לעם סגלה מכל העמים אשר על פני האדמה:
You are the children of Hashem your God; you shall not cut yourselves or make a bald spot between your eyes for the dead. For you are a holy people to Hashem your God, and Hashem has chosen you to be for Him a treasured people from all peoples that are upon the face of the earth.
In this passage, God forbids us from engaging in self-mutilation as an expression of mourning and grief when someone passes away. (Such practices were commonplace in many ancient cultures, and still exist today among some groups.) The Torah tells us that to engage in such mourning practices somehow contradicts the idea that we are "the children of Hashem" and a "holy people to Hashem." What is the connection between these two concepts?
In his commentary on the Torah, Rav Avraham ibn Ezra (d.1167) writes that the message here is that God loves us even more than a father loves his children, and therefore we should trust Hashem like a small child trusts his father, even when he cannot understand his father's actions, that no matter what happens, everything God does is truly for our benefit.
The Ramban (Rav Moshe ben Nachman, d.1270) cites the explanation of the ibn Ezra, and adds that part of the underlying message here is that, as the children of God and His treasured people, we should have absolute confidence in the reality of the afterlife. For this reason, excessive acts of grief and mourning, such as self-mutilation, are inappropriate, for they imply that the loss of our loved ones is absolute. However, the Ramban continues, the Torah does not prohibit crying over the death of a loved one, for it is natural for those who love each other to cry upon their separation, even in life.
Rabbi Mattisyahu Salomon, the famous mashgiach of the Lakewood yeshiva, expanded upon this idea in a speech (printed in With Hearts Full of Faith, p.75):
A father and mother send off their son to a yeshivah in Israel. They are the ones that encouraged him to go. They helped him choose the yeshivah, and they are the ones paying for it. Nevertheless, when they take their son to the airport and say good-bye, they cry. Why do they cry? Isn't everything going exactly as they wanted it to go? Why does a mother cry at her daughter's wedding? Because it is human nature to cry at times of parting with a loved one.
When a loved one dies, it is a moment of parting, not only for a certain period of time but for as long as we live on this earth. This is the pain we are allowed to feel. This is the pain that we are supposed to feel. It is right to feel the loss of a departed loved one, and it is right to give expression to that loss with our tears. But excessive grief? That is forbidden. Did we ever see parents mutilating themselves and tearing out their hair in the airport when they are sending off their children to study in a distant land? Not very likely. Self-mutilation expresses something much deeper than the pain of parting. It expresses the shock at the immensity of the tragedy and horror at coming face-to-face with evil. These have no place at a Jewish death.